Koichi Tohei Shihan on Ame no Torifune (i)…

At count One, push your [koshi] forward as though to push your one point forward horizontally and thrust your arms forward forcefully, keeping the wrists bent. The upper body is kept vertical, bending neither forward nor backward. The arms are not thrust forward so much as brought forward with the feeling of extending Ki from the [koshi]. The right leg should be stretched easily to the rear.

At count Two, draw back your [koshi], at the same time pulling back your wrists to your hips. It should be a pulling back with the [koshi] and not a pulling back with the arms. The right leg then is slightly bent and the left leg is straightened.

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Bear in mind that this exercise is more for the [koshi] than for the arms.